1. Field of the Invention
The present invention relates to an optical scanner reflecting/deflecting a light beam such as a laser beam for scanning an object.
2. Description of the Background Art
In general, a two-dimensional image apparatus such as a laser printer or a scanner is mounted with an optical scanner precisely scanning an object with a laser beam. This type of optical scanner reflects/deflects the laser beam with a light deflector such as a galvanometer mirror or a polygon mirror for scanning an objective surface of a photosensitive drum or the like. While the light deflector rotates at an equiangular velocity, the laser beam must scan the objective surface at a uniform rate. Therefore, the optical scanner employs an f-θ (ef-theta) lens as an optical system letting the laser beam reflected/deflected by the light deflector scan the objective surface at a uniform rate. The f-θ lens is an optical system having a distortion characteristic satisfying y=fω (f: focal distance, ω: half angle of view) in relation to an ideal image height y.
FIGS. 40 and 41 show a conventional optical scanner mounted with an f-θ lens 104. FIG. 40 is a schematic block diagram of the optical scanner developed along a Y-Z plane, and FIG. 41 is a longitudinal sectional view developing the optical scanner shown in FIG. 40 along an optical axis. Referring to FIGS. 40 and 41, numeral 100 denotes a light source (semiconductor laser), numeral 101 denotes a collimator lens, numeral 102 denotes a cylindrical lens, numeral 103 denotes a polygon mirror, numeral 104 denotes the f-θ lens, numeral 105 denotes an anamorphic lens and numeral 106 denotes an objective surface. Directions X, Y and Z shown in FIGS. 40 and 41 are perpendicular to each other.
The light source 100 oscillates a laser beam 107 directly modulated by a driving circuit (not shown). This laser beam 107 is parallelized by the collimator lens 101 and converged by the cylindrical lens 102 for forming a linear image on a reflecting surface 103r of the polygon mirror 103. The polygon mirror 103 rotates about a rotational axis 103c by tens of thousands of revolutions per minute and the f-θ lens 104 is an optical system converting equiangular velocity motion of incident light from the reflecting surface 103r to uniform motion, whereby a light beam reflected by the reflecting surface 103r of the polygon mirror 103 is deflected at an equilateral velocity and scans the objective surface 106 in the direction Y. The anamorphic lens 105 converges light incident from the f-θ lens 104 perpendicularly (direction X) to a primary scanning direction (direction Y) for forming an image on the objective surface 106.
As shown in FIG. 40, the light beam scans the objective surface 106 over a scanning line length W, and hence the f-θ lens 104 must have a wide total angle θ of view. Further, the size of an image has recently been so increased that an optical scanner having a large scanning line length W is required. Assuming that f represents the focal distance of the f-θ lens 104 at the working wavelength for the light beam, the following relational expression holds:W=fθWhen the scanning line length W is enlarged while keeping the total angle θ of view constant, therefore, the focal distance f of the f-θ lens 104 is increased. In order to enlarge the scanning line length W while keeping the focal distance f of the f-θ lens 104 constant, on the other hand, the total angle θ of view must be increased. In this case, the aperture of the f-θ lens 104 is so increased that it is difficult to precisely work the f-θ lens 104 and correct optical aberration values thereof, to readily increase the cost for the f-θ lens 104.
Compactification of the optical scanner has also been required in recent years. As shown in FIG. 42, an f-θ lens 104 built in the optical scanner is formed by three groups of lenses, i.e., a first lens 111 having negative refracting power, a second lens 112 having positive refracting power and a third lens 113 having positive refracting power. Between the total length L (face-to-face distance between an entrance-side curved surface 111i of the first lens 111 and an exit-side curved surface 113e of the third lens 113) of the f-θ lens 104 and a focal distance f, the following relational expression holds:0.100≦L/f≦0.108Hence, the total length L exceeds 0.100×f. An f-θ lens having optical performance not deteriorated also when the total length L is further reduced has recently been required.
When the light source 100 is formed by a semiconductor laser, the oscillation wavelength of the semiconductor laser varies with the operating temperature. The imaging position of the light beam varies with change of the oscillation wavelength due to chromatic aberration of the f-θ lens 104, to result in such a problem that no stable beam spot is obtained on the objective surface 106 and resolution of the image is reduced. When the light output of the semiconductor laser is steeply increased, for example, the operating temperature of the semiconductor laser is abruptly increased and the wavelength of the laser beam shifts to a longer side. After the light output is stabilized, the operating temperature is reduced also in a laser burning state and hence the wavelength shifts to a shorter side. Further, the semiconductor laser exhibits such mode hopping that the central wavelength discontinuously varies with temperature change or instability of causing a large number of longitudinal modes at a low output. When the optical scanner has a wide range of application, the semiconductor laser must be oscillated between low and high outputs and hence it is required to improve chromatic aberration of the f-θ lens 104 to form a stable beam spot on the objective surface 106. When employing a semiconductor laser having a working wave range of 400 nm to 410 nm, for example, chromatic aberration of magnification of about 0.2 mm is caused on an end of an angle of view and it is difficult to obtain high resolution in the conventional f-θ lens 104 having the three groups of lenses 111, 112 and 113.